Chem 1B
Chapter 13 Exercises
Name:________________________
Exercises #1
1.
Consider the following reaction: CO(g) + H2O(g)  CO2(g) + H2(g)
(a) In one experiment 1.0 mole of H2O and 1.0 mole of CO are placed in a sealed flask and heated to
350oC. In the second experiment 1.0 mole of H2 and CO2 are placed in another sealed flask of the
same volume and heated to 350oC. After equilibrium is reached in both cases, is there any difference
in the composition of the mixtures in the two flasks? Explain.
(Answer: no different – a given reaction has one value of K at the given temperature. For this
reaction, which has equal number of moles of products and reactants, if the starting concentrations are
the same, there should be same concentrations of the components in the equilibrium mixture,
regardless of the direction of the initial reaction to reach equilibrium.)
(b) If some 14C-labeled CO molecules are introduced into the equilibrium mixture in the above
reaction (without disturbing the equilibrium, will the 14C be found only in the CO or will some be
found in the CO2? Explain.
(Answer: 14C will be found in both CO and CO2 because equilibrium is a dynamic process – both
forward and reverse reactions continue to occur at equilibrium.)
2.
(a) Write the expressions for the equilibrium constant Kc for the following equilibrium systems.
(i) CH4(g) + H2O(g)  CO(g) + 3 H2(g)
(ii) N2(g) + 3 H2(g)  2 NH3(g)
(iii) (NH4)2CO3(s)  CO2(g) + 2 NH3(g) + H2O(g)
(iv) NH3(g) + HCl(g)  NH4Cl(s)
(v) HNO2(aq) + H2O(l)  H3O+(aq) + NO2-(aq)
(b) For equations (i) – (iv), write the expressions for the equilibrium constant Kp.
3.
An equilibrium mixture of H2, N2, and NH3 gases at 500 K is found to contain\ the following
concentrations: [H2] = 1.197 M, [N2] = 0.399 M, and [NH3] = 0.203 M. (a) Determine the equilibrium
constant Kc and Kp for the following reaction at 500 K. (R = 0.08206 L.atm/mol.K)
N2(g) + 3H2(g)  2NH3(g)
(b) What are the values of Kc and Kp for the given reversed reaction at 500 K?
NH3(aq)  ½ N2(g) + 3/2 H2(g)
(c) If the concentration of N2 is increased by 1.000 mol/L, in which direction will the net reaction occur to reach
new equilibrium position? How would the concentrations of H 2 and NH3 change in the new equilibrium
mixture? Will each of them increase or decrease or not change in the new equilibrium?
(Answer: (a) Kc = 0.0602; Kp = 3.58 x 10–5; (b) Kc’ = 0.245; Kp’ = 5.98 x 10–3;
(c) For reaction in (a), net reaction to the right; [H2] decreases and [NH3] increases in the
new equilibrium)
1
Chem 1B
Chapter 13 Exercises
4.
Name:________________________
Explain the following statement: “for a given reaction at a fix temperature, there is only one value for
the equilibrium constant (Kc or Kp), but there will be an infinite number of equilibrium positions.”
(Answer: a given reaction has one value of Kc or Kp at a given temperature, but the compositions at
equilibrium can be different.)
5.
What is the difference between equilibrium constant (Kc or Kp) and reaction quotient (Qc or Qp)?
(Answer: equilibrium constant is a value derived equilibrium constant expression using concentration
values at equilibrium; whereas reaction quotients are values derived from the same expression using
concentrations that are not necessarily the equilibrium concentrations. Values of reaction quotients
are often used to determined whether or not a given reaction mixture is at equilibrium.)
6.
Consider the following data obtained for the reaction: N2(g) + 3H2(g)  2NH3(g) at 500 K.
Initial Concentration, M
Concentrations at Equilibrium, M
Expt. # [N2]
[H2]
[NH3]
[N2]
[H2]
[NH3]
——————————————————————————————————
1
1.000
1.000
0
0.921
0.763
0.157
2
1.000
3.000
1.000
0.938
2.816
1.123
3
0
0
1.000
0.399
1.197
0.203
——————————————————————————————————
Show that, at constant temperature the equilibrium constant Kc is the same regardless of the
composition of the equilibrium mixture.
(Answer: Each of the above experiments yield Kc = 0.0603; a given reaction has one value of
equilibrium constant regardless of how the reaction is carried out.)
7.
Consider the reaction: N2(g) + 3H2(g)  2NH3(g).
A mixture containing 1.000 mole of N2 and 3.000 moles of H2 are placed in a 10.0-L reaction vessel
and allowed to react at 523 K. When equilibrium is established, the concentration of NH3 was found
to be 0.0112 mol/L. What are the concentrations (in mol/L) of N2 and H2 at equilibrium? Calculate
the equilibrium constants Kc and Kp for the above reaction at 523 K. (R = 0.0821 L.atm/(K.mol))
(Answer: Kc = 0.0585; Kp = 3.18 x 10–5)
8.
The reaction: N2(g) + 3H2(g)  2NH3(g) has Kp = 3.6 x 10-5 at 500 K.
A mixture contains H2, N2, and NH3 such that their partial pressures are 2.76 atm, 0.92 atm, and 0.16
atm, respectively. Determine if the mixture is at equilibrium? If not, predict in which direction will
the net reaction occur to reach equilibrium?
(Answer: Qp = 1.3 x 10–3 > Kp; the mixture is not at equilibrium; net reaction is to the left)
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Chem 1B
Chapter 13 Exercises
Name:________________________
Exercises #2
1.
Given the following equilibrium:
N2(g) + ½ O2(g)  N2O(g);
Kc = 2.4 x 10-18
N2(g) + O2(g)  2 NO(g);
Kc = 4.1 x 10-30
Calculate the equilibrium constant for the following reactions:
2.
(a) N2O(g) + ½ O2(g)  2 NO(g);
(Answer: Kc = 1.7 x 10–12)
(b) 4 NO(g)  2 N2O(g) + O2(g).
(Answer: Kc = 3.4 x 1023
Given the equilibrium constants for the following reactions at a certain temperature,
(a) CO(g) + H2O(g)  CO2(g) + H2(g);
(b) CO(g) + 2 H2(g)  CH3OH(g);
Kc = 1.0 x 105
Kc = 1.4 x 107
Calculate the equilibrium constant for the following reaction at the same temperature.
CO2(g) + 3 H2(g)  CH3OH(g) + H2O(g)
3.
(Answer: Kc = 1.4 x 102)
The reaction: H2(g) + I2(g)  2HI(g) has Kc = 55 at 350oC.
A mixture containing 1.000 mole each of H2 and I2 is introduced into an evacuated 1.00-L reaction
vessel and sealed. When the reaction reaches equilibrium at 350oC, what are the molar concentrations
of H2, I2, and HI, respectively, in the equilibrium mixture.
(Answer: [H2] = [I2] = 0.79 M and [HI] = 1.6 M)
4.
The reaction: COCl2(g)  CO(g) + Cl2(g) has Kc = 1.3 x 10-4 at a certain temperature.
If the initial concentration of COCl2 was 0.500 M and there were no CO or Cl2, what are the
concentrations of COCl2, CO, and Cl2 when the reaction reaches equilibrium?
(Answer: [COCl2]e = 0.492 M, and [CO] = [Cl2] = 8.1 x 10–3 M)
5.
Nitric oxide, a significant air pollutant, is formed from the reaction of nitrogen and oxygen at high
temperature, such as in an automobile engine. The reaction is endothermic:
N2(g) + O2(g)  2NO(g);
Ho = 180 kJ
(a) At constant temperature, does the equilibrium amount of nitric oxide formed increase, decrease, or
stays the same if pressure is increased as a result of compressing the mixture? Explain.
(b) Would more nitric oxide be formed if the engine block becomes overheated? Explain.
(Answer: (a) [NO] remains the same if pressure is increased; equilibrium does not shift in either
direction because of equal number of reactant and product molecules in gaseous state.
(b) Yes. More No will be formed (equilibrium shifts to the right) if temperature increases in an
endothermic reaction.)
3
Chem 1B
Chapter 13 Exercises
Name:________________________
Exercise #3
1.
Nitrosyl bromide, NOBr, decomposes according to the following equation:
2 NOBr(g)  2 NO(g) + Br2(g)
A 0.64 mole sample of nitrosyl bromide is placed in an evacuated, sealed 1.00-L reaction vessel.
When the reaction reaches equilibrium at a certain temperature, the concentration of NOBr in the
equilibrium mixture is 0.46 M. Calculate the concentration of NO and Br2 at equilibrium, and the
equilibrium constant Kc.
(Answer: [NO] = 0.18 M; [Br2] = 0.090 M, and Kc = 0.014)
2.
When 1.000 mole of PCl5 is introduced into a 5.000 L container at 500 K, 78.50% of the PCl5
dissociates to give an equilibrium mixture of PCl5, PCl3, and Cl2. Calculate the values of Kc and Kp at
500 K for the following equilibrium.
PCl5(g)  PCl3(g) + Cl2(g),
3.
(Answer: Kc = 0.573; Kp = 23.5)
For the equilibrium: PCl5(g)  PCl3(g) + Cl2(g),
Kp = 1.42 at a certain temperature.
If the initial partial pressures are PPCl5 = 3.00 atm, PPCl3 = 2.00 atm, and PCl2 = 1.50 atm, what are the
equilibrium partial pressures of PCl5, PCl3, and Cl2?
(Answer: at equilibrium, PPCl5 = 2.76 atm, PPCl3 = 2.24 atm, and PCl2 = 1.74 atm)
4.
The decomposition of solid ammonium carbamate, NH4(CO2NH2), to gaseous ammonia and carbon
dioxide is an endothermic reaction.
NH4(CO2NH2)(s)  2 NH3(g) + CO2(g)
(a) When a solid sample of NH4(CO2NH2)(s) is introduced into an evacuated flask at 25oC, the total
pressure of the gas at equilibrium is 0.116 atm. What is the value of Kp at 25oC?
(Answer: at equilibrium, PNH3 = 0.0773 atm and PCO2 = 0.0387 atm, and Kp = 2.31 x 10–4 )
(b) Given that the decomposition reaction is at equilibrium, how would the following changes affect
the total quantity of NH3 in the flask once equilibrium is re-established? Briefly explain your answer.
(i) Adding CO2
(ii) Adding solid NH4(CO2NH2)
(iii) Removing CO2
(iv) Adding nitrogen gas
(v) Increasing the total volume
(vi) Increasing the temperature
(Answer: (i) equilibrium shifts left – NH3 decreases; (ii) adding solid has no effect on equilibrium;
(iii) equilibrium will shift right and [NH3] increases; (iv) No effect – N2 is not part of the system;
(v) Increasing total volume will shift equilibrium to the right – quantity of NH3 increases, but its
partial pressure and concentration will not change.)
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Chem 1B
Chapter 13 Exercises
5.
Name:________________________
The equilibrium constant Kc for the following reaction is 7.20 x 10-2 at 25oC
NH4HS(s)  NH3(g) + H2S(g)
A sample of NH4HS is placed in an evacuated 1.00 L container and the above reaction is allowed to
reach equilibrium at 25oC. What are the partial pressures of NH3 and H2S at equilibrium?
(Answer: PNH3 = PH2S = 6.6 atm)
6.
The commercial production of hydrogen gas by steam-reformation of methane gas involves an
endothermic reaction represented by the following equation.
CH4(g) + H2O(g)  CO(g) + 3H2(g); Ho = 206 kJ
Use Le Chatelier’s principle to explain what would happen to the amount of hydrogen gas at
equilibrium when:
(a) carbon monoxide is removed
(b) water vapor is removed
(c) methane gas is removed
(d) carbon monoxide is added
(e) the pressure is increased by pumping argon gas into the equilibrium mixture.
(f) A valve that leads the gaseous mixture into a second reaction chamber is opened, causing the
pressure to drop.
(g) a catalyst is added to the equilibrium mixture.
(h) the temperature of the equilibrium mixture is increased
(Answer: (a) Equilibrium shifts right and [H2] increases; (b) Equilibrium shifts left and [H2] decreases;
(c) Equilibrium shifts left and [H2] decreases; (d) Equilibrium shifts left and [H2] decreases;
(e) No effect – Argon gas is not part of equilibrium system;
(f) Equilibrium shifts right and [H2] increases;
(g) No effect – catalyst increases both forward and reverse reaction;
(h) This is an endothermic reaction – the forward reaction absorbs heat. Equilibrium shifts right if
temperature increases and [H2] increase.
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